In recent years, as image-recording materials, materials for forming color images have been particularly predominant and, specifically, recording materials for an inkjet system, recording materials for a thermal transfer system, recording materials for an electrophotographic system, transfer type silver halide light-sensitive materials, printing inks, and recording pens have found widespread use. Also, in photographing devices such as CCDs for photographing equipment, and in LCDs and PDPs for display, color filters are used for recording or reproducing a color image. In these color image recording materials and color filters, colorants (dyes or pigments) of three primary colors of a so-called additive color mixing process or subtractive color mixing process have been used in order to display or record full-color images. In actuality, however, there is no fast colorant having the absorption characteristics capable of realizing a preferred color reproduction region and resisting various use conditions and environmental conditions. Thus, the improvement thereof has strongly been desired.
Dyes or pigments to be used for the above-mentioned uses are required to have in common the following properties. That is, they are required to have absorption characteristics favorable in view of color reproduction and have good fastness under the conditions of the environment wherein they are used, for example, fastness against light, heat, and an oxidative gas such as ozone. In addition, in the case where the colorant is a pigment, the pigment is further required to be substantially insoluble in water or in an organic solvent, to have a good fastness to chemicals, and not to lose the preferred absorption characteristics it shows in a molecularly dispersed state even when used as particles. Although the required properties described above can be controlled by adjusting the intensity of intermolecular mutual action, both of them are in a trade-off relation with each other, thus being difficult to allow them to, be compatible with each other.
Besides, in the case of using a pigment as the colorant, the pigment is additionally required to have a particle size and a particle shape necessary for realizing desired transparency, to have good fastness under the conditions of the environment wherein they are used, for example, fastness against light, heat, and an oxidative gas such as ozone, to have good fastness to an organic solvent and chemicals such as a sulfurous acid gas, and to be capable of being dispersed in a used medium to a level of fine particles, with the dispersed state being stable. In particular, those pigments have been strongly desired which have a good hue, show a high tinctorial strength in light, heat and moisture and in an active gas present in the environment, and is fast against light.
That is, in comparison with a dye which is required to have properties as dye molecules, the pigment is required to have more properties, i.e., it is required to satisfy all of the above-mentioned requirements as a solid of an aggregate of a colorant (dispersion of fine particles) as well as the properties as molecules of a coloring material. As a result, a group of compounds which can be used as pigments are extremely limited, in comparison with dyes. Even when high-performance dyes are converted to pigments, few of them can satisfy requirement for the properties as a dispersion of fine particles. Thus, such pigments are difficult to develop. This can be confirmed from the fact that the number of pigments registered in Color Index is no more than 1/10 of the number of dyes.
Azo pigments are excellent in hue and tinctorial strength which are characteristics of coloring, and hence they have widely been used in printing inks, inks for an inkjet system, and electrophotographic materials. Of the pigments, the most typically used azo pigments are yellow diarylide pigments and red naphthol azo pigments. Examples of such diarylide pigments include C.I. pigment yellow 12, C.I. pigment yellow 13, and C.I. pigment yellow 17. Examples of the naphthol azo pigments include C.I. pigment 208 and C.I. pigment 242. However, these pigments are inferior in fastness, particularly light fastness, and hence they are decomposed when prints printed by them are exposed to light, thus being inappropriate for prints which are to be stored for a long time.
In order to remove such defects, there have been disclosed azo pigments having a fastness improved by increasing molecular weight or by introducing a group having a strong intermolecular mutual action (see, for example, patent documents 1 to 3). However, even the improved pigments, for example, the pigments described in patent document 1 have the defect that they have still insufficient light fastness though improved to some extent, and pigments described in, for example, patent documents 2 and 3 have a greenish hue and a low tinctorial strength, thus being inferior in coloring characteristics.
Also, patent documents 4 and 5 disclose colorants which have absorption characteristics of excellent color reproducibility and have a sufficient fastness. However, all of the specific compounds described in the patent documents are soluble in water or in an organic solvent, thus being insufficient in fastness to chemicals.
In the case of expressing a full-color image based on the subtractive color mixing process using three colors of yellow, magenta, and cyan or using four colors further including black, use of a pigment having an inferior fastness as a one color pigment, gray balance of the prints would be changed with the lapse of time, and use of a pigment having inferior coloring characteristics would reduce color reproducibility upon printing. Thus, in order to obtain prints which can maintain high color reproducibility for a long time, there have been desired a pigment and a pigment dispersion which have both good coloring characteristics and good fastness.
Azo colorants have conventionally been utilized as colorants in various fields since they often have various visible light absorptions. For example, they have come into use in various fields such as coloration of synthetic resins, printing inks, colorants for sublimation type thermal transfer materials, inks for inkjet recording, and colorants for color filters. Major performances required for azo colorants as colorants include an absorption spectrum. Hue of a colorant exerts a great influence on color tone and feeling of a body colored with the colorant, giving a large effect on visual sensation. Therefore, there have long been made studies on absorption spectrum of a colorant.
Conventionally known azo dyes containing a nitrogen-containing, 5-membered ring as an azo component are also disclosed in patent documents 6 and
On the other hand, many of typical organic pigments are polymorphic and, in spite of having the same chemical formulation, such pigments are known to take two or more crystal forms.
Of organic pigments, some organic pigments such as azo pigments can form fine and size distribution-controlled particles by selecting appropriate reaction conditions upon synthesis thereof, and there are pigments such as copper phthalocyanine green which are formed into pigments by allowing extremely fine and aggregated particles produced upon synthesis to grow in a subsequent step with size distribution being controlled, and pigments such as copper phthalocyanine blue pigment which are formed into pigments by pulverizing coarse and uneven particles produced upon synthesis in a subsequent step and controlling the size distribution. For example, a diketopyrrolopyrrole pigment is generally synthesized by reacting a succinic diester with an aromatic nitrite in an organic solvent (see, for example, patent document 8). The crude diketopyrrolopyrrole pigment is heat-treated in water or in an organic solvent, and then subjected to pulverization such as wet milling into a form appropriate for use (see, for example, patent document 9). For example, with a diketopyrrolopyrrole pigment of C.I. Pigment Red 254, an α-type crystal form and a β-type crystal form are known (see, for example, patent document 10). Also, with an azo pigment of C.I. Pigment Yellow 181, several crystal forms are known (see, for example, patent document 11).    Patent document 1: JP-A-56-38354    Patent document 2: U.S. Pat. No. 2,936,306    Patent document 3: JP-A-11-100519    Patent document 4: JP-A-2005-213357    Patent document 5: JP-A-2003-246942    Patent document 6: JP-A-55-161856    Patent document 7: JP-A-2002-371214    Patent document 8: JP-A-58-210084    Patent document 9: JP-A-5-222314    Patent document 10: JP-A-8-48908    Patent document 11: US Patent Application Publication No. 2008/058531